Polyester resin impregnating and coating solutions and their use

ABSTRACT

The polyester resin impregnating and coating solution comprises polyesters having structures of the formula (I)                    
     where n is from 0 to 10 
     and where the solution is essentially free from monomers containing acrylic, vinylic or allylic unsaturation and where the solvent employed preferably comprises aliphatic saturated C 2-6  alcohols, C 3-6  ketones or C 3-6  carboxylic esters or mixtures thereof, the proportion of the solvent being from 5 to 60% by weight of the overall polyester resin impregnating and coating solution.

This is the U.S. national stage of international applicationPCT/EP98/01043 and is filed under 35 U.S.C. 371.

The invention relates to polyester resin impregnating and coatingsolutions, to their use for coating shaped articles, and tocorresponding processes for the coating of shaped articles.

It is necessary to provide the windings and surfaces of electricalcomponents with electrically insulating coatings for electricalinsulation and for protection against external influences such as rain,splashes of water, dust, salts, solvents or mechanical influences. Thesecoatings can be produced by impregnation or coating with electricallyinsulating resins. At the present time, polyester resin compositions areprincipally employed for these applications.

In this context it is common to employ unsaturated polyesters asimpregnating resin in the form of a solution in a copolymerizablemonomer, generally styrene. Following the coating of the shaped articlesor components with the polyester resin compositions they are cured bycopolymerization with the monomers, especially styrene.

DE-A-31 07 450 describes unsaturated polyesters of this kind containingcyclopentadiene oligomers as end groups. They are composed of maleicacid as unsaturated acid component and are employed in the form ofsolutions in styrene for the production of shaped articles and coatings.

DE-A-32 30 924 describes processes for preparing unsaturated polyesterresins based on maleic anhydride as unsaturated acid component, theresulting polyesters being reacted with dicyclopentadiene, andesterification being carried out in the presence of N-hydroxyalkylimidesof monounsaturated cycloaliphatic 1,2-dicarboxylic acids. The polyestersare dissolved in styrene.

EP-B-0 118 786 and EP-B-0 260 688 describe processes for preparingmolding materials from unsaturated polyester resins, where esters, basedon maleic anhydride as a saturated acid and reacted withdicyclopentadiene, are dissolved in styrene and cured in a two-stagecuring process using two different free-radical initiators.

DE-A-1 570 273 and DE-A-1 720 323 describe unsaturated polyesters havingcyclic imide groups. These polyesters are also employed as a solution instyrene.

In addition, impregnating varnishes are employed. They comprisedissolved resins, which are frequently based on natural oils and resinsand which may also be chemically modified and combined with syntheticpolymers, such as alkyd-epoxy resins or phenolic resins. For processingthey are dissolved in solvents with typical concentrations of about 50%.The solvents employed here are predominantly aromatic hydrocarbons, suchas toluene or xylene, alone or in combination with aliphatic orcycloaliphatic hydrocarbons, such as white spirit. In this case, onlylow concentrations are obtained in these solutions. Moreover, a highlevel of safety expenditure is necessary because of the solvents used.

When the above-described polyesters are employed as impregnating resinsin the form of solutions in copolymerizable monomers, such as acrylates,allyl phthalate, styrene, methylstyrene or methyltoluene orvinyltoluene, or as impregnating varnishes in the form of solutions inaromatics, some of these monomers or aromatics are released in thecourse of the use of the mixtures for coating. Known applications ofimpregnating compositions with these substances are accompanied bylosses in mass of about 20 to 30%. These considerable amounts must beremoved from the workplace, since the monomers or aromatics are in manycases injurious to health and irritant to the skin, and thus constitutea health hazard to those working with these materials. The amounts ofmonomer or aromatics drawn off by suction are generally disposed of inwaste-air incinerators, possibly giving rise to unwanted emissions.Furthermore, the substances lost in this way represent a considerableeconomic loss. Furthermore, there is a risk that the monomers will notbe completely copolymerized in the course of curing. Residual monomersand aromatics remaining in the cured compositions may escape, especiallyfrom electrical insulation compositions, which generally become hotduring use, and can cause odor pollution or damage to health. Themonomers may also undergo after curing in the compositions, as a resultof which they may undesirably alter the service properties of thecompositions.

It is an object of the present invention to provide polyester resinimpregnating and coating solutions (impregnating varnishes) which avoidthese disadvantages and which, in particular, have a high solids contentand an acceptable solvent.

We have found that this object is achieved in accordance with theinvention by a polyester resin impregnating and coating solutioncomprising polyesters having structures of the formula (I)

where n is from 0 to 10 and where the solution is essentially free frommonomers containing acrylic, vinylic or allylic unsaturation.

It has been found in accordance with the invention that theabovementioned unsaturated polyesters or polyester resins can be curedeven without the use of the monomers containing acrylic, allylic orvinylic unsaturation which have hitherto been regarded as absolutelynecessary. The compositions are essentially free from these monomers.The term “essentially” means here that there are no amounts of monomerscontaining acrylic, allylic or vinylic unsaturation that substantiallyalter the properties of the polyester resin compositions. The amount ofmonomers containing acrylic, allylic or vinylic unsaturation ispreferably not more than 30, particularly preferably not more than 10and, in particular, not more than 5% by weight based on the overallweight of the polyester resin compositions. With particular preferencethe polyester resin compositions are free from monomers containingacrylic, allylic or vinylic unsaturation. It has been found that, byusing solvents other than compounds containing acrylic, allylic orvinylic unsaturation, it is possible to obtain highly concentratedpolyester resin solutions. The content of solvents is preferably from 5to 60, particularly preferably from 8 to 20, in particular from 10 to15% by weight, based on the overall weight of the polyester resinsolution. The term “solvent” here means those solvents or diluents whichare preferably partially or completely free from aromatics which do notenter into any chemical reactions, especially in the course of curing.These are compounds which during or after curing escape from thepolyester resin compositions or remain in them without entering intochemical bonds to the polymer structure. Monomeric or oligomericcompounds carrying functional groups which allow them to be reacted inthe course of curing of the polyester resin are not embraced by the term“solvent”. As solvents it is preferred to employ aliphatic saturatedC₂₋₆ alcohols, C₃₋₆ ketones or C₃₋₆ carboxylic esters or mixturesthereof It is particularly preferred to employ aliphatic saturated C₂₋₄alcohols, such as ethanol n-propanol, isopropanol, n-butanol, isobutanoland tert-butanol. The polyester resins employed in accordance with theinvention are highly soluble in these solvents, preferably inconcentrations of more than 70% by weight, with particular preferencemore than 75% by weight, and in particular, more than 80% by weight. Thesolvents dry well so that the polyester resin compositions, afterapplication, can easily be freed from the solvents. The health hazarddue to the preferred solvents is low compared to aromatic solvents. Thesolutions here are of low viscosity, rendering them suitable asimpregnating and coating solutions for the coating of shaped articles.

The shaped articles coated by the novel process are preferablyelectronic or electrical components or carrier materials for electricalinsulators, especially flat electrical insulators. Examples of suchshaped articles or components are wires, coils, motor windings,transformer windings and other components. Insulators which can be usedinclude carrier materials for sheet-like insulating materials, such asglass fibers, mica tapes and other absorbent materials, and alsocombinations thereof, and in this context one option is to terminatecuring of these materials at the B-stage in order to obtain curableprepregs. Curing is ended when the prepregs have solidified to an extentwhere they are not stuck together and can be stacked or wound.

The polyester resin solutions employed in accordance with the inventionare impregnating, casting or coating solutions. The process according tothe invention for coating shaped articles comprises the generally knownprocesses of dip impregnation, the trickle technique, the dip rollingprocess, the flooding process and the process of casting for theimpregnation of windings. These processes can if desired be assisted bythe use of reduced pressure and/or superatmospheric pressure. Suitableprocesses are known to the skilled worker. The invention also provides aprocess for coating shaped articles by impregnating in, casting with orcoating with polyester resin solutions, as defined above, removing thesolvent and thermally and/or photochemically curing the polyester resincoating.

The polyester resin solutions employed in accordance with the inventioncan be heated in the process for coating in order to reduce theviscosity and to facilitate their application. The polyester resinsolutions employed in accordance with the invention can be processed onknown installations with little or no modification.

The polyester resin compositions employed in accordance with theinvention in the polyester resin solutions comprise unsaturatedpolyesters. These polyester resins can be synthesized by known processesfor the preparation of polyesters, generally by polycondensation ofpolyfunctional hydroxy compounds with polyfunctional acids and/or theiranhydrides at relatively high temperatures. It is often advantageous tostart from the esters of such substances and to obtain the polyesters bytransesterification at relatively high temperatures, since in some casessuch transesterifications proceed more readily and more rapidly thandirect esterification. In addition, it is also possible to usepolyfunctional amines as well, in which case polyesters having amidestructures are obtained. The use of monofunctional starting materials isanother possibility, for example in order to regulate the molecularweight. All known polyester resins can be employed in accordance withthe invention provided they comprise at least partially unsaturatedpolyesters.

Examples of basic structural units from which the polyesters can bebuilt are:

adipic acid, suberic acid, phthalic acid isomers, tetrahydrophthalic,endomethylenetetrahydrophthalic, hexahydrophthalic, fumaric, maleic,itaconic, citraconic, trimellitic and pyromellitic acid, ethyleneglycol, polyethylene glycols, propylene glycol, polypropylene glycols,butanediol isomers, hexanediol, neopentylglycol, trimethylolpropane,glycerol, pentaerythritol, bisphenol A, hydrogenated bisphenol A,OH-polyfunctional polymers, such as hydroxyl-modified polybutadienes orhydroxyl-carrying polyurethane prepolymers and epoxy resins,polyfunctional natural substances or derivatives thereof, such aslinseed oil fatty acid, dimeric and polymeric linseed oil fatty acid,castor oil, castor oil fatty acid.

The introduction of amide and imide structures is described, forexample, in DE-A-1 570 273 and DE-A-1 720 323. Such polyester amides orpolyester imides are employed, for example, if there are particularrequirements in respect of thermal stability.

The polyesters employed in accordance with the invention are unsaturatedoverall. In this case the double bonds are preferably provided by thestructures below of the formula (I) and if desired by unsaturateddicarboxylic acids or their anhydrides, such as fumaric, maleic,itaconic or citraconic acid or their anhydrides or mixtures thereof As aresult, the unsaturated polyesters are based on the structures of theformula (I) and, if desired, these acids as unsaturated compounds.

The polyester resin compositions employed can additionally comprisesaturated polyesters. Saturated polyesters have no C—C double or triplebonds. Aromatic unsaturation and the double bond in the 5-membered ringof the structures of the formulae (I) and (II) are not regarded asdouble bond in this case since they do not participate in an additionpolymerization. Examples of saturated acids in such polyesters arephthalic acids in the various isomeric forms.

The polyester resin compositions preferably contain not more than 50,particularly preferably not more than 20% by weight of saturatedpolyesters, based on the overall weight of the polyester resincompositions without solvent.

The unsaturated polyester have structures of the formula (I)

where n is from 0 to 10. n here can be an integer or else may denote amean value. Preferably, n is from 0 to 7, particularly preferably from 0to 5 and, in particular, from 0 to 3. Preferred examples of n are 0, 1,2. The structures of the formula (I) are based on cyclopentadiene (CPD)or dicyclopentadiene (DCPD). Processes for preparing these structuresare known, for example, from M. C. Kloetzel; Org. Reactions 4 (1948) 1to 59 or W. M. Carmody; Ind. Eng. Chem. 30 (1938) 245 to 251. Thestructures may have an oxygen atom on the free valence. Structures withn>0 are preferably produced in situ by a graft reaction of CPD or DCPD,respectively, on structures with n=0 at more than 130° C., preferablymore than 170° C.

The polyesters may also preferably have structures of the formula (II)

where n is from 0 to 10. The preferred ranges of n are as indicatedabove. The structures of the formula (I) can be prepared by reactionwith carboxylic acids. These carboxylic acids are preferably present inthe polyester or are introduced as polycarboxylic acids into thepolyester.

The polyesters preferably have structures of the formula (III)

where n is from 0 to 10. In this case, all possible isomeric structurescan be present at the ethylenic double bond. The preferred ranges for nare as indicated above. The structures of the formula (II) and (III) canbe prepared by reacting maleic anhydride with dicyclopentadiene orsimilar compounds. This produces monosubstituted maleic acids, whichtherefore have a free acid function. These compounds are linked with thepolyesters. The structures of the formula (II) and (III) are preferablyin the form of esterification products.

Structures of the formulae (I) and/or of the formula (II) and/or of theformula (III) can also, in addition, be present in the form of monomericor oligomeric compounds in the polyester resin solutions. In this casethe structures are preferably in the form of esterification products. Inthis case esterification can take place with monohydric alcohols and/orpolyhydric alcohols and/or alkoxylation products thereof and/orpolyether polyols and/or polyester polyols. They can be obtained, forexample, by reaction with polyether polyols or polyester polyols ofpolyethylene oxide, polypropylene oxide, polytetrahydrofuran and/orpolycaprolactone. Examples of preferred alcohols are neopentylglycol,propylene glycol, dimethylolethane, cyclohexanediol, 1,6-hexanediol,trimethylolpropane, diethylene glycol monoethyl ether, and ethoxylationproducts or propoxylation products thereof It is possible in particularto use an ethoxylation product of 1 mol of trimethylolpropane and 20 molof ethylene oxide. Through the nature of the alkylating agents and thedegree of alkoxylation it is also possible to control properties of theend products such as hardness, hydrophilicity and elasticity. It is alsopossible to carry out only partial reaction of such polyols withstructures of the formulae (I) to (III), the remaining hydroxyl groupseither remaining free or possibly being esterified or etherified withother compounds or reacted with other compounds that are reactive withhydroxyls. Examples of suitable such compounds are isocyanates orepoxides, and also hydroxyl-containing natural oils such as castor oil.

Preferred monomeric and oligomeric products are obtained by reactingequal molar proportions of dicyclopentadiene and maleic anhydride in thepresence of water and then reacting the products with polyols, thenumber of OH groups in the polyols corresponding to the number of freeacid functions of the substituted maleic acid. Particularly preferredalcohols in this case are 1,6-hexanediol, trimethylolpropane with 20 EO(ethylene oxide) and diethylene glycol monoethyl ether.

The structures of the formulae (I) to (HI) can also be present in thecorresponding monomeric and oligomeric compounds in the form of amide oramine bonds. These compounds can be prepared, for example, by reactionwith mono- and polyfunctional amines.

The products of these reactions can be salt-like adducts, althoughamides are preferred. Examples are the reaction products ofamino-functional polyethylene oxides, polypropylene oxides or dieneoils.

Dihydrodicyclopentadienol, corresponding to the structure of the formula(I) with a hydroxyl on the free valence, is industrially obtainable.Accordingly, ester structures can also be introduced by esterifyingmono- or polycarboxylic acids or carboxyl functions of the polyesterswith this alcohol.

The structures of the formulae (I) to (III) can be introduced into thepolyesters as end group substituents after the polyesters have beenprepared or can be employed already as additional monomer components inthe reaction mixture used for preparing the polyesters. In this case thestructures can be attached at different sites on the polymer backbone.When used as end group substituents, the proportion of structures whichcan be introduced in this way is limited. Especially in the case of highmolecular mass polyesters the additional use of the monomeric oroligomeric compounds is advantageous for increasing the reactivity.

Cyclopentadiene can be grafted onto the double bonds of the unsaturatedpolyesters employed in accordance with the invention to giveendomethylenetetrahydrophthalic acid structures. This is particularlythe case when maleic or fumaric acid is employed.

It is preferred to employ polyesters containing from 5 to 40,particularly preferably from 5 to 10% by weight of these structures. Thepolyester resin compositions preferably contain from 50 to 100,particularly preferably from 70 to 100, and in particular, from 90 to100% by weight of monomeric or oligomeric or polymeric compounds whichhave the structures of the formulae (I) and/or (II) and/or (III).

In this way it is possible to achieve good curing even with polyesterswhich have few unsaturated sites without the need to add monomerscontaining acrylic, allylic or vinylic unsaturation. Moreover, it isnecessary to add only small amounts of solvents in order to obtain anappropriate viscosity.

Following the impregnation, casting or application as a coating of thepolyester resin solutions (impregnating varnishes) employed inaccordance with the invention, and after removal of the solvent, thermaland/or photochemical curing is carried out. For this purpose theimpregnated varnishes preferably include compounds or functional groupswhich can be thermally and/or photochemically activated in order toinitiate crosslinking or free-radical polymerization. These initiatorscan be attached chemically to the polyesters or can be present as freecompounds in the impregnating varnishes. Examples of thermallyactivatable initiators are compounds which form free radicals whenheated. Known free-radical initiators are peroxides, azo compounds,azides and C—C-labile compounds. A considerable acceleration of thecuring and/or reduction in the curing temperature is possible when metalcoinitiators are used, such as compounds of cobalt, of manganese, ofiron, of nickel or of lead. The polyester resin compositions accordingto the invention are highly UV-sensitive in the presence of UVinitiators of the α-cleaving type (Norrish Type I) or of theH-donor/acceptor systems (Norrish Type II). A preferred mode ofintroducing H-acceptor groups in this case is the concomitant use ofphenone compounds which can be incorporated by condensation, for examplehydroxy- or bishydroxybenzophenone, in the course of thepolycondensation of the polyester resins. In this case thephotoinitiators are activated with actinic radiation, preferably TVradiation. Other suitable photoinitiators have xanthone structures,thioxanthone structures and/or the above phenone structures. Thephotoinitiator is preferably hydroxybenzophenone, which is incorporatedby condensation into the polyesters. Furthermore, the introduction ofbenzophenone structures by way of benzophenone di- and/or-tetracarboxylic acid derivatives, preferablybenzophenonetetracarboxylic dianhydride, is possible with preference.

In this context, curing can take place in one or more steps. Forexample, curing can be carried out first with actinic radiation andsubsequently or simultaneously with peroxides or C—C-labile substances.It is also possible to carry out partial curing, with complete curingfollowing at a later point in time. Suitable initiators and curingtechniques are set out in the documents described at the outset.

Preferably, after removal of the solvent, the polyester resin solutions(impregnating varnishes) are first of all cured on the surface with UVlight and then cured to completion using thermally activatableinitiators with heating. Also of importance is a process in which thepolyester resin solutions, for example in electrical windings, are firstof all cured in the interior of the components by means of heat, whichis generated by the flow of an electric current through the component,and then are after-cured, if desired, on the surface with UV light. Anydesired combination and sequence of the above-mentioned methods can beused for curing.

The viscosity of the polyester resin solutions employed in accordancewith the invention can be adjusted not only by mixing with the solventbut also by mixing together different polyesters. Preferably, monomericor oligomeric compounds as well as are added which have structures ofthe formulae (I)/(II)/(III). For instance, it is possible to usepolyesters having a relatively high melt viscosity and a high softeningpoint in the present invention, and to establish the desired lowviscosity of the solvent and these compounds. These compounds cantherefore be termed “reactive diluents”, but do not have thedisadvantages of the known ethylenically unsaturated compounds such asstyrene.

The polyester resin solutions preferably have a viscosity of below 5000mPas at 25° C., more preferably less than 3000 mPas. They are preferablystable in terms of viscosity for at least 24 hours at a temperature atwhich they have a viscosity of not more than 2000 mPas, particularlypreferably not more than 500 mPas. Owing to the special reactivity ofthe dicyclopentadienyl structures in the monomeric, oligomeric orpolymeric compounds having structures of the formulae (I), (II) or(III), it is possible to provide polyester resin solutions which arecatalyzed ready for reaction and which can be processed in liquid formwithout the use of known unsaturated monomers such as styrene,vinyltoluene, α-methylstyrene, allyl esters and (meth)acrylic esters.

However, it is also possible to make additional use of these knownmonomers containing acrylic, allylic or vinylic unsaturation, in smallamounts, for example in order to formulate low-styrene polyester resinsolutions which are of low viscosity. Consequently it is possible, forexample, to formulate low-styrene polyester resin solutions which meetstatutory limits on styrene concentrations or styrene emissions.Preferably, none of these reactive diluents is present.

Where specific requirements are imposed on the polyester resin coatings,for example on the hardness or elasticity, the composition of thepolyesters in the polyester resin compositions can be adaptedcorrespondingly. For example, the chain length of the polyols orpolycarboxylic acids can be varied. Polyester resins composed withhexanediol or adipic acid, for example, are more flexible than polyesterresins based on phthalic acid and ethylene glycol. In addition, it isknown to control the properties by the concomitant use of polyfunctionalcompounds, which produce branches in the polyester molecules. Knowncompounds are trimellitic acid and trimethylolpropane.

The polyester resin compositions or solutions can be prepared by anydesired methods. Preferably, the compounds which regulate the reactivityand viscosity, and have structures of the formulae (I) and (II) or(III), are prepared separately and then mixed with the polyesters, thesolvent and any other compounds used. In many cases it is also possibleby appropriate adjustment of the stoichiometric proportions to preparein situ nonpolyester compounds which regulate the reactivity andviscosity in the course of the polyester preparation.

The polyester resin solutions used in accordance with the invention may,moreover, include further ingredients customary for polyester resins,such as catalysts, color-imparting compounds, pigments, fillers andother auxiliaries.

The polyester resin solutions used in accordance with the invention canbe employed as impregnating, casting or coating solutions for thecoating of shaped articles or films. Corresponding impregnating, castingor coating techniques are known to the skilled worker. Examples ofshaped articles and components are wires and wound articles, such ascoils, motor windings, transformer windings or corresponding film-basedcomponents.

The invention is illustrated in more detail below by way of examples.

EXAMPLES Example 1

A stirred flask fitted with heater and distillation device is chargedwith 317.1 g (2.1 mol) of dihydrodicyclopentadienol, 292.3 g (2.0 mol)of adipic acid, 101.3 g (1.0 mol) of 1,6-hexanediol and 0.7 g ofdibutyltin dilaurate (DBTL). The mixture is heated rapidly to 120° C.under a gentle stream of nitrogen. The temperature is gradually raisedto 190° C. over 4 hours, during which the water of condensation whichforms is distilled off A resin having an acid number of 11 andviscosities of 1540 mPas at 25° C. and 260 mPas at 50° C. is obtained.

Example 2

A stirred flask fitted with a heater and reflux condenser is chargedwith 661.1 g (5.0 mol) of dicyclopentadiene and 490.3 g (5.0 mol) ofmaleic anhydride. The mixture is heated to 125° C. under a gentle streamof nitrogen, and then 95.0 g (5.0 mol+5 g) of water are added through adropping funnel over the course of one hour. The mixture is left toreact at 125° C. for one hour. A monocarboxylic acid as represented inthe formula (III) is formed, where the free valence carries a hydroxylgroup and n is zero. The contents of the flask are cooled to 70° C., and245.15 g (2.5 mol) of maleic anhydride, 557.15 g (5.5 mol) of1,6-hexanediol, 4.0 g of dibutyltin dilaurate (DBTL) and 0.5 g ofhydroquinone are added. The mixture is heated rapidly to 120° C. under agentle stream of nitrogen. Then the temperature is gradually raised to190° C. over the course of 6 hours, during which the water ofcondensation which forms is distilled off A highly viscous resin havingan acid number of 18 and viscosities of 7840 mPas at 50° C. and 2016mPas at 75° C. is obtained.

Example 3

A stirred flask fitted with a heater and a reflux condenser is chargedwith 1586.52 g (12.0 mol) of dicyclopentadiene and 1176.7 g (12.0 mol)of maleic anhydride. The mixture is heated to 125° C. under a gentlestream of nitrogen, and then 226.00 g (12.0 mol+10 g) of water are addedthrough a dropping funnel over one hour. Reaction is allowed to continueat 125° C. for one hour. This gave the monocarboxylic acid of theformula (III) as obtained in Example 2. Then the contents of the flaskwere cooled to 70° C., and 715.0 g (6.0 mol) of 1,6-hexanediol, 4.0 g ofdibutyltin dilaurate DBTL) and 0.5 g of hydroquinone were introduced.The mixture is heated to 120° C. under a gentle stream of nitrogen andthe temperature is then gradually raised to 190° C. over 6 hours, withthe water of condensation formed being removed by distillation. A softresin having an acid number of 24 and viscosities of 3650 mPas at 50° C.and 944 mPas at 75° C. is obtained.

Example 4

A stirred flask fitted with a heater and a reflux condenser is chargedwith 661.1 g (5.0 mol) of dicyclopentadiene and 490.3 g (5.0 mol) ofmaleic anhydride. The mixture is heated to 125° C. under a gentle streamof nitrogen, and then 95.0 g (5.0 mol+5 g) of water are added through adropping funnel over one hour. Reaction is allowed to continue at 125°C. for one hour. This gives the monocarboxylic acid of the formula (III)described in Example 2. Then the contents of the flask are cooled to 70°C., and 1859.0 g of TP 200 (TP 200 is an ethoxylation product of onemole of trimethylolpropane and 20 mol of ethylene oxide) (correspondingto 5.5 mol equivalent of OH), 3.00 g of dibutyltin dilaurate (DBTL) and0.3 g of hydroquinone are introduced. The mixture is then heated rapidlyto 120° C. under a gentle stream of nitrogen and the temperature is thengradually raised to 190° C. over 6 hours, with the water of condensationformed being removed by distillation. A highly viscous, fluid resinhaving an acid number of 21 and viscosities of 9340 mPas at 25° C. and1560 mPas at 75° C. is obtained.

Impregnating varnishes

The compounds of Examples 1 to 4 can be diluted to give clearformulations—to an unlimited extent with lower ketones such as acetone,methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) and at leastto a concentration of 70% by weight with C₂₋₆ alcohols. Using mixturesof ketones and alcohols it is possible to obtain even greater dilutions,although this is generally not necessary since the solutions reach thedesired viscosities from a concentration of about 80% by weight. It isconsequently possible to obtain highly concentrated impregnatingvarnishes and coating materials.

3 impregnating varnishes were prepared having the compositions indicatedbelow. The parts are by weight.

Impregnating varnish 1

10 parts of resin from Example 1,

80 parts of resin from Example 2,

10 parts of resin from Example 3,

25 parts of isopropanol and

4 parts of tert-butyl perbenzoate are mixed to give a clear solution oflow viscosity (86 s/DIN 4).

Impregnating varnish 2

10 parts of resin from Example 1,

80 parts of resin from Example 2,

10 parts of resin from Example 4,

25 parts of isopropanol and

4 parts of tert-butyl perbenzoate are mixed to give a clear solution oflow viscosity (93 s/DIN 4).

Impregnating varnish 3

10 parts of resin from Example 1,

80 parts of resin from Example 2,

10 parts of resin from Example 4,

25 parts of isopropanol,

4 parts of tert-butyl perbenzoate and

3 parts of Lucirin® BDK (BASF) are mixed to give a clear solution of lowviscosity (91 s/DIN 4).

The impregnating varnishes are applied to degreased metal panels using adoctor blade with a gap height of 100 μm. This corresponds to a dry filmthickness of 55 μm. The panels are then conditioned at 130° C. for 2hours in a convection oven. Brownish, clearly transparent, hard coatingsare obtained which are resistant to methyl ethyl ketone.

The panels coated as above are also conditioned at 80° C. for 3 hours ina convection oven. The coatings obtained are tacky. The panels aresubsequently irradiated for 2 minutes using a medium-pressure mercurylamp having an energy of 80 mW/cm². In this case the surface tack isslightly reduced in the case of impregnating varnishes 1 and 2, where inthe case of impregnating varnish 3 a brownish, clearly transparent, hardcoating is obtained which is resistant to methyl ethyl ketone.

Samples of the ready-to-use impregnating varnishes prepared as indicatedabove are stored in sealed glass bottles at room temperature for oneyear, at 40° C. for one month and at 60° C. for one week. In all casesneither the viscosity nor the curing properties are changed after thisstorage.

We claim:
 1. A polyester resin impregnating or coating solutioncomprising polyesters having the structures of the formula (I)

where n is from 0 to 10, and a solvent, and where the solution containsnot more than 5% by weight of monomers containing acrylic, vinylic orallylic unsaturation, based on the total weight of the solution.
 2. Asolution as claimed in claim 1, wherein the solvent employed comprisesaliphatic sated C₂₋₆ alcohols, C₃₋₆ ketones or C₃₋₆ carboxylic esters ormixtures thereof.
 3. A solution as claimed in claim 1, wherein theproportion of the solvent is from 5 to 60% by weight of the overallsolution.
 4. A solution as claimed in claim 1, wherein the polyestershave structures of the formula (II)

where n is from 0 to
 10. 5. A solution as claimed in claim 4, whereinthe polyesters have structures of the formula (III)

where n is from 0 to
 10. 6. A solution as claimed in claim 5, whichcomprises monomeric or oligomeric compounds containing structures offormula (I).
 7. A solution as claimed in claim 6, wherein the monomericor oligomeric compounds are based on mono- or polyhydric alcohols whichare esterifed with the structures of the formula (III).
 8. A solution asclaimed in claim 1, which comprises photoinitiators and/or thermallyactivatable initiators for curing the polyester resins.
 9. A process forcoating shaped articles or films by impregnation in, casting with thepolyester resin solution as defined in claim 1, removing the solvent andthermally and/or photochemically curing the polyester resin coating. 10.A solution as claimed in claim 1, wherein the solution is free ofmonomers containing acrylic, vinylic or allylic unsaturation.
 11. Asolution as claimed in claim 1, wherein the solvent comprises analiphatic saturated C₂₋₄ alcohol.
 12. A solution as claimed in claim 1,wherein the solvent is free from aromatics.
 13. A solution as claimed inclaim 1, wherein the proportion of the solvent is from 8 to 20% byweight of the overall solution.
 14. A solution as claimed in claim 1,wherein the solvent consists of aliphatic saturated C₂₋₆ alcohols, C₃₋₆ketones or C₃₋₆ carboxylic esters or mixtures thereof, and wherein theproportion of the solvent is from 5 to 60% by weight of the overallsolution.